Within the communications industry, there is an ever increasing need for higher performance portable devices having long battery lives. For example, handheld personal information devices (e.g., palmtop computers), cell phones, pagers, and the like, are processing data at faster rates, performing more sophisticated functions, and storing larger amounts of data, while simultaneously functioning for increased periods of time on internal battery power. For example, it is not uncommon for modern cell phone devices to operate continuously in standby mode for several days on end.
Low-power integrated circuits are critical to extend functioning on internal battery power for such handheld devices. To extend battery life, many handheld devices are designed to enter a standby mode when there full functionality is not required by the user. For example, a cell phone is designed to enter a standby mode when it is not being used in a voice conversation. The cell phone can “wake up” from standby when a call is received or when the user desires to place a new call. Similarly, many personal information devices are designed to enter standby mode after some duration of non-use from the user, and wake up when the user activates some function, accesses some data (e.g., clicks a GUI icon) etc. While in standby mode, modern battery power devices are designed to require minimal amounts of power, thereby extending their battery lives.
Well-designed standby mechanisms can greatly extend the functional life of a portable battery powered device. Many standby mechanisms function by turning off one or more circuit blocks of the device to save power during periods of nonuse, and subsequently restarting the one or more blocks when the device returns to operational mode. Accordingly, the design of integrated circuits that implement standby modes, turning off blocks and later turning on those blocks for full functionality, is an area of great interest to the electronics industry. It is important that those mechanisms which turn off and subsequently turn on circuit blocks draw minimal amounts of current. Additionally, is important that such mechanisms reliably wake up the device upon some external event, such as, in the case of a cell phone, receiving an incoming phone call.
Devices are also required to reliably power up from an off state, or unpowered state, in addition to waking up from a standby mode. When a device is initially powered up, it is important that the first voltages applied to energize the elements of the device are stable and orderly. For example, voltage transients, voltage spikes, and the like, can cause different circuit elements to power on out of order from one another, leading to problems. Such transients can be especially difficult for an analog circuit. Analog circuits can be more vulnerable to current and/or voltage transients than digital circuits. Hence, it is desirable that the startup mechanism to wake up from an off state or standby mode function reliably in the presence of noise or other disturbances on the power supply, and provide a smooth predictable startup current/voltage to reliably wake up the device.
Specific circuits have been designed to ensure the overall device reliably starts up from an off state (or standby mode). Such circuits are referred to as startup circuits. Startup circuits are used in powering up devices from a power off condition in addition to waking up devices from sleep modes.
Thus, what is required is a startup circuit which maintains a more constant, non-varying startup current over a range of power supply voltage level, in comparison to the prior art. What is required is a startup circuit having very low static power consumption. Additionally, what is required is a startup circuit that will reliably produce the required amount of startup current in order to reliably power up or wake up an integrated circuit. The present invention provides a novel solution to the above requirements.